I want to use the new --image-extra-size flag[1] but my use-case
calls for megabyte granularity of this value. Rather than adding
60% to an 800MB image, maybe I only want to add 100 or 200MB, etc.
[1] https://review.opendev.org/#/c/655127/
Change-Id: I8fb9685d60ebb1260d5efcf03c5c23c561c24384